1. Field of the Invention
The present invention relates to a color conversion definition creating method of creating a color conversion definition defining an association between a color in a color representation area of an arbitrary device and a color in a color representation area of a predetermined target device, and a color conversion definition creating program storage medium storing a color conversion definition creating program for creating a color conversion definition.
2. Description of the Related Art
There is known various types of input devices for obtaining image data through input of an image, for example, a color scanner for reading a recorded image to obtain image data and a digital still camera (DSC) in which image data is obtained in such a manner that an image of a subject is formed on a solid state imaging device and then read. In those input devices, the image data is represented by data of a predetermined range of for example 0 to 255 on each of three colors for example red (R), green (G) and blue (B). Colors, which can be represented by numerical values in the predetermined range on each of those three colors of R, G and B, are restricted as a matter of course. For this reason, even if colors of the original image have very plentiful representation, conversion of the colors into image data once using such an input device causes an image represented by the image data to be restricted to the colors in a certain color representation area in the R, G and B color space.
Also with respect to an output device for outputting an image in accordance with image data, there are known various types of output devices, for example, a photographic printer in which a printing paper is exposed by a laser beam to develop the printing paper so that an image is recorded on the printing paper, a printer in which an image is recorded on a sheet in accordance with a system such as an electrophotographic system and an inkjet system, a printing machine in which a rotary press is rotated to create a number of printed matters, and an emissive display device such as a CRT display and a plasma display in which light is emitted on a display screen in accordance with image data to display an image. Also with respect to an output device, similar to the above-mentioned input device, there exists a color reproduction area according to the associated output device. That is, the output device is able to represent various colors in accordance with for example image data representative of three colors of R, G and B or image data representative of four colors of C (cyan), M (magenta), Y (yellow) and K (black). However, colors, which can be represented, are restricted to a certain color representation area (for example, a color representation area represented by numerical values of a range of 0 to 255 on each of colors of R, G and B) in an output device color space (for example, an RGB space and a CMYK space). The color representation area in the above-mentioned input device and output device is referred to as a color gamut.
Recently, there is increased a tendency that image data is used on a common basis between the input device and the output device, and there is known a way of keeping a color of an image to be equivalent between the input device and the output device.
However, as mentioned above, since the color representation area is different in the respective devices, the use of image data on a common basis according to the way of keeping a color of an image to be equivalent may bring about an omission in color representation on a portion in which color representation areas are not coincident with one another. A large omission involves unnaturalness in an image represented by image data used in a common basis.
On the other hand, it is empirically known that regardless of the fact that the color representation is different, it is possible to represent the originally same image as a natural image for person's eyes in the various devices. The natural images represented by the various types of devices are represented by colors mutually different somewhat in accordance with a difference in the color representation area of devices. However, a person may get the natural impression on images represented by any devices, since adaptability of person's eyes is high.
Thus, there is needed a color conversion for converting a color of an image to keep the natural impression for the image in the use of image data on a common basis. It is desired that such a color conversion is a color conversion that colors in a color representation area (a color gamut) of a certain device are completely associated with colors in a color representation area (a color gamut) of another device. This color conversion is referred to as a gamut mapping.
As the conventional color conversion (gamut mapping), there is known, for example, a technology that a common color space (a device-independent-data space), which is independent of a device, for example, an L*a*b* color space, is placed at the middle, and image data on a color space depending on an input device, which is obtained by the input device, is converted into image data on a common color space to perform a gamut mapping on the common color space, and then the image data subjected to the gamut mapping is converted into image data on a color space dependent on an output device (cf. for example, Japanese Patent Application Laid Open Gazette Sho. 60-105376, Japanese Patent Application Laid Open Gazette Sho. 61-288662, and Japanese Patent Application Laid Open Gazette Hei. 4-196675).
Further, in view of a matter that the finally necessary image data is image data in a color space (an output color space) depending on a predetermined device, there is also proposed such a technology that image data is converted into image data on the color space depending on the device, and data exceeding the range of for example 0 to 255 on each of R, G and B on the color space (e.g. the RGB space) are subjected to a compression processing that negative data is clipped to 0 and data exceeding 255 is clipped to 255 so that data on each of R, G and B are compressed to the range of 0 to 255 (Japanese Patent Application Laid Open Gazette Hei. 2-214266 (compression as to the CMYK space), and Japanese Patent Application Laid Open Gazette Hei. 4-334267 (compression as to density)). This is a simple technology and corresponds to an example of a gamut mapping in a device-dependent color space.
It often happens that such a gamut mapping is performed in accordance with a color conversion definition defining the association between a color in a certain color representation area and a color in another color representation area. An appropriate color conversion definition makes it possible to implement a gamut mapping keeping a natural impression for an image. Such an appropriate color conversion definition is referred to as a high quality of color conversion definition.
There is proposed an algorithm for creating such a high quality of color conversion definition by a computer and the like. However, there is no almighty algorithm, and in order to create a high quality of color conversion definition, in many cases, there is a need of know-how and trial and error.
However, it is considered that an environment of needs of gamut mapping will be more expanded as computer and communication technology advance, and there is desired a color conversion definition creating method capable of readily creating a high quality of color conversion definition when it is needed.